Conventional carburetors for internal fuel combustion engines are known to have a fuel-and-air mixing passage for delivering a controlled ratio of fuel-and-air to the combustion chamber of a running two or four cycle engine. The mixing passage is defined by a body of the carburetor and has a venturi disposed between an upstream or air inlet region and a downstream or mixture outlet region of the passage. Generally controlling the amount of air flowing through the venturi is a butterfly-type choke valve disposed pivotally within the air inlet region of the mixing passage. During engine cold-start conditions, the choke valve is in a substantially closed position allowing only a small amount of air to flow through the mixing passage and thus creating the needed rich mixture of fuel-and-air for easy engine cold starts. Otherwise, during warm engine starts and warm running engine conditions, the choke valve is substantially open creating minimal air flow restriction. Generally controlling the flow rate of the fuel-and-air mixture flowing through an intake manifold to the combustion chamber of a running engine is a butterfly-type throttle valve, which is disposed within the mixture outlet region of the mixing passage. As the throttle valve rotates from a substantially closed position to a wide open throttle position, and with the choke valve in a substantially open position, the engine speed will increase from idle to maximum or full power.
Typically, a pressure differential measured between a substantially constant pressure fuel metering chamber of a metering assembly and the high vacuum venturi region of the mixing passage causes liquid fuel to flow from the fuel metering chamber and into the venturi region via a fuel feed passage and a fuel nozzle disposed at a radially inward portion of the venturi or venturi region of the mixing passage. As air flow increases through the venturi, dictated by the position of the throttle valve and the air demand of the combustion engine, the venturi vacuum increases thus causing the fuel flow through the fuel feed passage and nozzle to increase. In this way, an engine initially at idle speed will increase in rpm to wide open throttle conditions with the increasing flow rate of the fuel-and-air mixture.
The fuel metering chamber is held at near atmospheric conditions and near constant volume by a flexible diaphragm disposed directly between the metering chamber and a reference chamber. The metering chamber is defined between a bottom side of the body of the carburetor and a top surface of the diaphragm. The reference chamber is defined between a bottom surface of the diaphragm and a bottom cover of the carburetor which carries an opening or nozzle that vents the reference chamber to atmosphere and/or filtered air. An integral or remote fuel pump, commonly operated via pressure pulses usually from the crankcase of the two cycle engine or the intake manifold of a four cycle engine, supplies fuel to the metering chamber via a supply valve which opens and closes in response to movement of the fuel metering diaphragm.
Of course, many other structural and dynamic factors of the carburetor contribute toward an easy start and smooth running engine which are also required to meet government and regulatory emission requirements. For instance, linkages are known to exist between exterior levers of the choke and throttle valves which make the positions of each valve inter-dependent to a limited degree. Moreover, a plurality of idle and intermediate speed fuel orifices are known to be orientated in the mixture outlet region of the mixing passage on either side of the throttle valve when closed. Such low speed orifices typically communicate with a fuel chamber which receives a controlled amount of fuel from the fuel metering chamber via a supplemental fuel passage. Usually the supplemental fuel passage is restricted controllably by a threaded needle valve which when rotated enters or retracts from the passage thus adjusting the ratio of fuel-to-air in the mixture for stable running conditions at low engine speeds.
Depending upon the engine type, displacement, and application, carburetors can become very complex, having highly machined and detailed bodies which incorporate many more numerous moving parts than those described above. All of this adds to the weight, manufacturing cost and maintenance expense of the carburetor. Likewise, there exist some two cycle engine applications, such as that of small lawn and garden appliances where a more simplistic, lighter, and less expensive carburetor would be ideal. Unfortunately, known carburetors must generally include all the costly components described above to support an easy start and reliable running engine which also meet regulatory emission requirements.